The present invention relates to an arrangement for perforating or cutting foils, particularly composed of a synthetic plastic material, which has a cutter and a countersupport.
Cutting or perforating of foils, particularly composed of a synthetic plastic material formed as thermoplastic synthetic plastic foils, which must form bags, sacks and the like, is difficult in practice inasmuch as a plurality of requirements must be satisfied. For manufacturing bags, sacks, and the like flexible containers of thermoplastic synthetic plastic foil, the connection of two foil layers by heated welding bars or wires, perforation or cutting of the foil layers is performed with a high number of cycles. The foil working machines run as a rule in two layers, i.e. uninterruptedly in a very long time period. This means that the cutter and the countersupport must have very long service life. When the cutter or the countersupport is worn out and must be replaced by a new device, this causes, in addition to the required replacement work, a stoppage of the entire manufacturing machine.
Great emphasis must be put on the quality of the cut or the design of a perforation of many small cuts. Thus the cut must be effective and clean for many types of foils, taking into consideration their chemical composition or physical properties or their thickness. Foils with different such properties have also different conditions during cutting. Therefore there is a further requirement that the perforating or cutting arrangement can always be effective for different properties of the foils.
It is known from practice that a foil is perforated or cut better when maintained under tension. Since the foils must be repeatedlY welded immediately near a perforation seam, and cooling of the weld seam results in shrinking, the tension increases, which can lead to weakening of the weld seam. The desire to hold the foil for perforating or cutting under tension and at the same time to keep it loose for good welding connection has not been satisfied. Many foils to be worked are stretched in one direction, and also additionally stretched many times in a transverse direction. At the same time, there are so-called linear foils which are very thin and difficult to perforate and cut. On the other hand, on the grounds of cost economy, it is desirable to work very thin-walled foils. These, however, are very difficult to perforate or cut. Finally, there is a reciprocatable cutter with an immovable counterwelding support, and also a rotary cutter with a rotary countersupport.
In dependence upon the different properties, particular physical properties of the foil, and also in dependence upon the working steps following a perforation step, it is necessary in practice to retain during perforation webs of greater or lesser width between small cuts. For attaining the desired and exact dimensions, a perforation cutter is provided with appropriately dimensioned teeth which determines the length of the individual partial cuts. The teeth have at least rectangular shape. There are also triangular teeth with front tips. It is thereby possible, by selection of the insertion depth in the counterwelding support, for example of rubber, to determine the length of the cut. However, it has been recognized that this is very difficult to attain in practice.
U.S. Pat. No. 3,456,540 discloses an arrangement for perforating foils of thermoplastic synthetic plastic material, which has a cutter arranged under the foil to be cut and provided with teeth, wherein the cutter is raisable in direction to the foil to be cut with perforations via a cylinder-and-piston unit. Bars are arranged above the foil and extend in a direction of elongation of the perforation cutter. A plurality of levers arranged one behind the other are coupled with the bars and provided with a helical spring and small roll which is arranged at its front end facing toward the perforation cutter and cooperates with the latter. For perforating the foil, the bars move in the direction of elongation of the perforation cutter and simultaneously thereto, so that the rollers are supported on the cutter with interposition of the foil. Thereby only such perforation cuts can be produced the individual cuts of which correspond to the length of the teeth of the perforation cutter. Changes to this length of the perforation cut are not possible, and they can be made when required, only by replacing the perforation cutter by a new perforation cutter with differently dimensioned teeth. Since the rollers are spaced from one another by great distances which extend over the length of three teeth of the perforation cutter, the bars must reciprocate over a relatively great distance.